The reason I have adopted a 5pin output is to bring out a ground connection to also support single ended headphones (headphones wired with a common ground) from a single jack (assuming a bridged balanced output). Team 5-pin!

Everyone to his liking. The 5-pin solution will suit all.
And it is also size compatible with a Neutrik 6.35mm stereo phone jack socket.
So no changes in mechanical parts required.
We thought long about all these before starting.

oh sure, I just dont quite understand that solution, I cant think of another situation where you would need to cater for both 3 wire and 4 wire cables on the 1 amplifier and if you have this connector and socket on the amplifiers/headphones, you have either built/modified them yourself, or changed the connectors.

So this means the only situation it would help with, would be people who are willing to reterminate their headphones that have a common ground wire, but are not willing to recable them; every other situation can be handled with 4 pin and suitably wired amp/headphone

and honestly I think the world needs another balanced headphone 'standard' like it needs another Pol Pot....

sorry I dont mean to be a pita about this, people will do what they please as you say. I've made this unholy proliferation of different balanced headphone 'standards' a bit of a pet cause and I get on my soap box every time a new one is introduced. We now have something like 6 different connectors in use because everyone wants to be individual, thats ridiculous....

it means I end up with jobs like this; which i'm not sure is a blessing or a curse... more work, but it means I have to keep more different connectors in stock.

anyway i'll let the thread get back on track, I know of quite a few people that have been waiting for this amp to be made more publicly digestible, I actually get messages about it on head-fi reasonably regularly. I built it up rough point to point (taylored) and like it a lot, but since I prefer balanced and all my sources are balanced, the circlotron version holds more interest for me.